The invention relates to a measuring device, particularly an encapsulated longitudinal measuring device, comprising a scale, a plurality of guide surfaces, and a scanning unit guided at the scale and urged resiliently against the guide surfaces. Such measuring devices are utilized primarily to measure the relative movement of mobile machine parts.
In the operation of machine tools equipped with such measuring devices, or measuring devices in general, oscillations and other accelerations are often produced which may influence the measurement. The elimination or at least the reduction of the effect of such oscillations and accelerations on an optical instrument is the problem on which the invention according to the German Offlegungschrift DT-OS No. 1,900,475 is based. An arrangement is described there for stabilizing a pair of binoculars held by an observer. To maintain the binoculars in the desired orientation about their pitching axis, a counterweight and a resilient guide are used, preferably with aperiodic attenuation.
Counterweight and attenuation members arranged to reduce the effect of oscillations and thrusts have also been used in other areas as well. However, such known methods for reducing the repurcussions of oscillations and thrusts cannot be simply applied to high precision longitudinal measuring devices, which are often capable of a precision of a few micrometers.
Due to the frequently rough operation of machine tools such measuring devices require special protection against external mechanical influences and special provisions for guiding mobile components. Generally such high precision measurement devices include a scanning unit which travels along the measuring scale. Both the scale and the scanning unit are surrounded by a tubular housing member which defines guide surfaces, and the scanning unit is positioned by rollers which travel along the guide surfaces. A powerful spring is used to bias the rollers against the guide surfaces and thereby prevent the scanning unit from lifting off the guide surfaces in the event of oscillations or accelerations of the tubular member.
For reasons of precision the scanning unit is guided in at least one plane directly at the measuring scale itself in many measuring devices. However, the scale must be protected against distorting compressive forces and friction against the scanning unit and, therefore, biasing forces acting to hold the scanning unit on the scale must be limited. Thus the danger exists that at peak acceleration of the tubular member the sensing unit may lift off the scale, thereby producing the possibility of both measuring errors and mechanical damage to the measuring device.